This invention relates to a transmission system and method both of which are for use in a data comunication network comprising an information processing unit, a communication control unit, a public telephone network, and a plurality of terminals, such as work stations or the like, connected through switched lines of the public telephone network.
In such a data communication network, the information processing unit is operable in accordance with a plurality of application programs. In this event, a session or communication is often carried out between hardware containing an application program and a selected terminal that may be called a destination terminal. In order to start or establish such communication between the application program and the destination terminal, the destination terminal is opened or initialized to be put into a communicable state prior to the communication. To this end, the information processing unit must transmit an opening command to the destination terminal when an opening request is issued from the application program.
More particularly, the information processing unit delivers a calling out or originating command to a communication control unit in response to the opening request. The calling out command accompanies a dial number or a calling data terminal equipment (DTE) address assigned to the destination terminal. Supplied with the calling out command, the communication control unit connects the destination terminal by the use of an automatic dialer, included therein, through the public telephone network and a network controller. After completion of such a connection, the communication control unit notifies the information processing unit of completion of the connection. As a result, a communication path is formed between the information processing unit and the destination terminal. Thus, the information processing unit first carried out the calling out operation of forming the communication path. After formation of the communication path, the information processing unit transmits the opening command through the communication control unit and the communication path to the destination terminal. Such an opening command is accompanied by a program address signal assigned to the application program in question and a terminal identifier assigned to the destination terminal. The destination terminal is put into a communicable state in response to the opening command. Thus, the information processing unit must carry out delivery of the opening command after the calling out operation.
As mentioned above, the information processing unit individually carries out the calling out operation and the delivering operation in a time division fashion on opening the destination terminal. Consequently, extra overhead inevitably exists in the information processing unit in the course of opening the destination terminal.
Such a transmission system is disclosed in Advanced Communications Function for VTAM issued by International Business Machines Corporation in 1980. In this prior art transmission system, the opening request is produced from an application program in the form of a macroinstruction specified by OPNDST (open destination) and is given to a program group which is called a VTAM (Virtual Telecommunications Access Method) program of the information processing unit. Responsive to the OPNDST, the information processing unit supplies the communication control unit with the calling out command which is named CONNOUT and which is accompanied by the dial number assigned to the destination terminal. The calling out command is processed by a network control program which runs in the communication control unit. After connection of the destination terminal, the information processing unit delivers the opening command referred to as BIND to the destination terminal through the communication control unit.